Feed chains are strategic components for satellite antennas. The current trend in communication satellites is toward multiple beam coverage, requiring more and more feed chains per satellite. This is particularly true for broadband applications at Ka-band, with current antenna designs having about 20 user-link dual-band (transmit and receive) feed chains per reflector in a single-feed-per-beam antenna configuration, and this number is expected to grow by at least a factor of two in the coming decade. This also means that the beam width has to become smaller. Current values of the beam width are in the range of 0.5 to 0.8 degrees and are expected to go down to a beam width of about 0.2 degrees.
In view of this trend, RFS systems used to improve beam pointing accuracy are of particular importance. One key component of RFS systems is the mode extractor at feed chain level, since the use of higher-order modes, such as the TM01 and TE21 (TE21) modes, enables refined pointing performance. These modes provide radiation patterns with a null on-axis, referred to as a “difference pattern” as illustrated in FIG. 1, in which the abscissa indicates an angle of observation in degrees at the location of the RFS system, and the ordinate indicates a signal strength in dB. In the figure, an example of the difference pattern 102 is represented by the dashed line, while an example of a “sum pattern” 101, typically produced by the fundamental modes TE11 (TE11), is represented by the solid line. The difference pattern enables more accurate pointing than the sum pattern. The RFS port extracts a signal from a beacon station that is usually located within the coverage in order to enable accurate beam pointing. However, this requires that at least one of the user-link feed chains per reflector antenna comprises a mode extractor for RFS purposes, i.e. a mode extractor capable of extracting a higher-order mode having a difference pattern.
In order to keep the feed cluster volume as small as possible, a very compact mode extractor for the RFS system is required. On the other hand, the mode extractor should also be compatible with a generic design for the user-link feed chain so that the mode extractor may be employed in different feed chains without having to adapt or customize the respective feed chain to the mode extractor beforehand.
In the prior art two main groups of configurations for feed chains in RFS systems, or more generally, feed chains comprising a mode extractor are known. The first group of configurations is based on the extraction of the TE21 mode with a mode extractor 201 arranged in between the antenna horn 203 and the user-link feed chain 202, as schematically illustrated in FIG. 2. This approach enables to use a generic feed chain design for the user-link, provided that the TE21 mode extractor 201 is designed so as to have limited impact on the fundamental modes over the two operating frequency bands (transmit and receive). An example of implementation is described in P. Lepeltier et al., “Thales Alenia Space France antennas: recent achievements and future trends for telecommunications”, in proceedings of the 2nd European Conference on Antennas and Propagation (EuCAP), pp. 1-5, 2007. According to this implementation, a coupling device is used to extract the TE21 mode. However, this requires a long coupling section and a specific combination network. In consequence, the mode extractor according to this implementation is ab aft the same size as a complete transmit/receive user-link feed chain.
J. C. Lafond et al., “Thales Alenia Space France antennas: recent achievements for telecommunications”, in proceedings of the 5th European Conference on Antennas and Propagation (EuCAP), pp. 3193-3197, 2011, illustrates the current trend to reduce feed chain length and diameter. A significant improvement is demonstrated for the user-link feed chain, the length of which is reduced by a factor of two. Naturally, this calls for at least a similar size reduction on the higher-order mode extractor.
The second group of configurations is based on the extraction of the TM01 mode with a mode extractor 301 arranged after the user-link feed chain 302 which is coupled to the antenna horn 303, as schematically illustrated in FIG. 3. An example of implementation is described in E. Amyotte et al., “High performance communications and tracking multi-beam antennas”, in proceedings of the 1st European Conference on Antennas and Propagation (EuCAP), 2006. This configuration enables the design of a very compact mode extractor. However, the main drawback is that a specific user-link feed chain design is required, i.e. a user-link feed chain design specifically adapted to the mode extractor 301. In particular, the receive section of the feed chain needs to be modified so as to propagate the required higher-order mode(s), and the septum polarizer, which is usually implemented in the receive chain for reasons of simplicity and compactness, has to be replaced by an orthomode junction (OMJ) associated with a specific combination network. In addition, in this implementation a mode filter is also required, resulting in a more complex feed chain design which is significantly different from the typical user-link only feed chain design. Thus, this approach requires a specific development for the feed chain including RFS, resulting in additional development costs.
The second group of configurations also includes those aiming at extraction of both the TM01 and TE21 modes. An example of implementation is provided in E. Reiche et al., “A dual circular combined K/Ka-band RF sensing feed chain for multi beam satellite antennas”, in proceedings of the 5th European Conference on Antennas and propagation (EuCAP), pp. 3198-3202, 2011. This approach still requires a specific user-link feed chain development with the additional design constraint that the TE21 mode also has to propagate in the receive part of the feed. This tends to degrade performance in comparison to a user-link only design, affecting in particular the transmit-to-receive rejection in the receive band which is typically obtained using below cut-off waveguide cross-section filtering.
Summarizing, current implementations of mode extractors are either bulky or require custom feed chain design. In addition, the implementations known in the art tend to have good extraction of the respective higher-order mode only over a limited frequency range, thus requiring the design to be tuned to a given beacon frequency.